Some dolphins appear to have orca friends. Scientists think they have figured out what’s going on

A pod of Pacific white-sided dolphins off the coast of British Columbia has been spotted collaborating with killer whales, a traditional enemy better known for catching great white sharks than friendly communication.

Scientists say they have documented how dolphins and a local population of killer whales, known as the Northern Resident Killer Whales, teamed up to hunt the orcas' primary food: salmon. Although other groups of killer whales feed on dolphins, the inhabitants of the North do not. However, the researchers reported that this is the first time this type of cooperative behavior has been documented between two marine mammals.

“Watching them dive and hunt in sync with dolphins completely changes our understanding of what these encounters mean,” said Sarah Fortune, chair of the Canadian Wildlife Federation's Great Whale Conservation and assistant professor of oceanography at Dalhousie University. Fortune was the lead author of the study, which was published Thursday in the journal Scientific Reports journal.

To witness interactions between dolphins and killer whales, the researchers filmed drone footage as well as underwater video by attaching suction cups to the killer whales, which were equipped with cameras and hydrophones.

Their footage showed the killer whales heading towards the dolphins and following them at surface level. Underwater footage showed that the killer whales also followed the dolphins during their dives to depths of up to 60 meters (197 feet), where the killer whales could hunt Chinook salmon.

Although light levels are low at these depths, Fortune reports that cameras captured killer whales catching salmon with clouds of blood spewing from their mouths, and hydrophones picked up the crunch of their prey.

To better understand what's going on, the researchers also listened to echolocation clicks made by dolphins and killer whales, which allow the animals to navigate and sense their environment by listening to the reflected echo of the noises they make. “We can look at the characteristics of these clicks to infer whether the whale is actively pursuing prey for fish, and whether it is likely to catch fish,” Fortune said.

Researchers recorded 258 cases of interaction between dolphins and killer whales between August 15 and August 30, 2020.

They found that all the whales that interacted with the dolphins also engaged in killing, eating, and searching for salmon.

Taken together, the data collected by Fortune and her colleagues suggests that killer whales, fearsome predators capable of taking on great whites and whale sharks several times their size, essentially used dolphins as scouts.

“Hunting with other animals that have echolocation, such as dolphins, can expand their acoustic field of view, providing more opportunities to detect where the salmon are. That's kind of the prevailing thought,” she explained. Using dolphins in this way will also allow killer whales to conserve energy, as salmon often hide in the deep to avoid predators such as killer whales.

But what do dolphins get from the interaction?

Video collected by Fortune and her colleagues showed that once the killer whales caught their kill and shared it with the pod, the dolphins quickly ate the leftovers.

But salmon is not a major part of the dolphins' diet, so greater access to food likely wasn't the only motivation, Fortune says. By interacting with orcas, the dolphins likely gain protection from other pods of orcas that pass through the area and prey on the dolphins.

In addition to the salmon-eating northern killer whales, the region is home to a special type of killer whale known as “Bigg” or transient killer whales, which specialize in eating marine mammals such as dolphins.

Interactions between northerners and dolphins have occurred off northeastern Vancouver Island for at least three decades, according to Brittany Wisona-Kelly, a senior manager at Canadian conservation group Ocean Wise's Whales Initiative. She was not involved in this study, but was studying the interactions between dolphins, porpoises and the same population of killer whales.

In her experience, it was the dolphins that initiated interactions with the killer whales, not the other way around, and she said she was skeptical that the two were actually engaged in foraging together. Instead, she said, the killer whales may have viewed the dolphins as annoying pests that were easier to deal with than to get rid of.

“Over several years of observation, we have concluded that it is dolphins and porpoises, not killer whales, who benefit most from these encounters. Dolphins and porpoises are likely receiving protection from their primary predator,” she said in an email.

“We hypothesize that northern killer whales do not clearly benefit from these interactions, but actively avoiding or resisting them may result in greater energetic costs than tolerating them,” she added.

Fortune, however, said her team's findings challenged the prevailing view among scientists about the interactions.

“Under this paradigm, dolphins would just hang out on the surface, nibble on scraps of food, without expending time, energy or effort in the process, which is certainly the case,” she said, adding that her team found no evidence of antagonistic or avoidant behavior in orcas towards dolphins.

What's more, Fortune and her colleagues' study was the first to use underwater imaging to understand behavior, she added.

Cooperation between different species is relatively common in nature, but less common among mammals and usually does not involve predators, says Judith Bronstein, a professor emeritus in the Department of Ecology and Evolutionary Biology at the University of Arizona who studies interspecies cooperation. However, she noted that coyotes have been spotted hunting with badgers And possums with ocelots.

Many species feed together, Bronstein said, noting that “mixed flocks of birds, mixed schools of fish, for example, look out for predators.”

“What's interesting about this example is that each species has different capabilities,” she said, “and when you look at cooperation between species, you're always looking for benefits that outweigh the costs.”

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